Background The rising prevalence of autism spectrum disorders (ASD) increases the need for evidence‐based behavioral treatments to lessen the impact of symptoms on children's functioning. At present, there are no curative or psychopharmacological therapies to effectively treat all symptoms of the disorders. Early intensive behavioral intervention (EIBI) is a treatment based on the principles of applied behavior analysis. Delivered for multiple years at an intensity of 20 to 40 hours per week, it is one of the more well‐established treatments for ASD. This is an update of a Cochrane review last published in 2012. Objectives To systematically review the evidence for the effectiveness of EIBI in increasing functional behaviors and skills, decreasing autism severity, and improving intelligence and communication skills for young children with ASD. Search methods We searched CENTRAL, MEDLINE, Embase, 12 additional electronic databases and two trials registers in August 2017. We also checked references and contacted study authors to identify additional studies. Selection criteria Randomized control trials (RCTs), quasi‐RCTs, and controlled clinical trials (CCTs) in which EIBI was compared to a no‐treatment or treatment‐as‐usual control condition. Participants must have been less than six years of age at treatment onset and assigned to their study condition prior to commencing treatment. Data collection and analysis We used standard methodological procedures expected by Cochrane. We synthesized the results of the five studies using a random‐effects model of meta‐analysis, with a mean difference (MD) effect size for outcomes assessed on identical scales, and a standardized mean difference (SMD) effect size (Hedges' g) with small sample correction for outcomes measured on different scales. We rated the quality of the evidence using the GRADE approach. Main results We included five studies (one RCT and four CCTs) with a total of 219 children: 116 children in the EIBI groups and 103 children in the generic, special education services groups. The age of the children ranged between 30.2 months and 42.5 months. Three of the five studies were conducted in the USA and two in the UK, with a treatment duration of 24 months to 36 months. All studies used a treatment‐as‐usual comparison group. Primary outcomes We found evidence at post‐treatment that EIBI improves adaptive behaviour (MD 9.58 (assessed using Vineland Adaptive Behavior Scale (VABS) Composite; normative mean = 100, normative SD = 15), 95% confidence interval (CI) 5.57 to 13.60, P < 0.001; 5 studies, 202 participants; low‐quality evidence; lower values indicate positive effects). We found no evidence at post‐treatment that EIBI improves autism symptom severity (SMD −0.34, 95% CI −0.79 to 0.11, P = 0.14; 2 studies, 81 participants; very low‐quality evidence). No adverse effects were reported across studies. Secondary outcomes We found evidence at post‐treatment that EIBI improves IQ (MD 15.44 (assessed using standardized IQ tests; scale 0 to 100, normative SD = 15), 95% CI 9.29 to 21.59, P < 0.001; 5 studies, 202 participants; low‐quality evidence) and expressive (SMD 0.51, 95% CI 0.12 to 0.90, P = 0.01; 4 studies, 165 participants; low‐quality evidence) and receptive (SMD 0.55, 95% CI 0.23 to 0.87, P = 0.001; 4 studies, 164 participants; low‐quality evidence) language skills. We found no evidence at post‐treatment that EIBI improves problem behaviour (SMD −0.58, 95% CI −1.24 to 0.07, P = 0.08; 2 studies, 67 participants; very low‐quality evidence). Authors' conclusions There is weak evidence that EIBI may be an effective behavioral treatment for some children with ASD; the strength of the evidence in this review is limited because it mostly comes from small studies that are not of the optimum design. Due to the inclusion of non‐randomized studies, there is a high risk of bias and we rated the overall quality of evidence as 'low' or 'very low' using the GRADE system, meaning further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate. It is important that providers of EIBI are aware of the current evidence and use clinical decision‐making guidelines, such as seeking the family’s input and drawing upon prior clinical experience, when making recommendations to clients on the use EIBI. Additional studies using rigorous research designs are needed to make stronger conclusions about the effects of EIBI for children with ASD.